Screwdown offset system and method for improved gauge control

ABSTRACT

A screwdown recalibration system is provided for use in a computer-controlled rolling mill. The computer uses the difference between gauge directly calculated by mass flow concepts and gauge calculated from a measured screwdown unloaded roll opening and roll force at each stand to recalibrate the screwdowns for that stand. The system is adaptable to either providing information to an automatic roll force gauge control system according to a predetermined schedule or to function as an online recalibration scheme which is an integral part of a more comprehensive roll force gauge control system.

United States Patent [72] Inventor Andrew W. SIIthJr.

Pittsburgh, Pl.

121 Appl. No. 872.461

[22| Filed Nov. 24. 1969 [45] Patented Aug. 24, 197] {73] Assignee Westinghouse flectrk Corporation Pittsburgh, PI. Continuation of application Ser. No.

677,308, Oct. 23, 1967, now MM- [541 SCREWDOWN OFFSET SYSTEM AND METHOD FOR IMPROVED GAUGE CONTROL 3,355,918 12/1967 Wa1lace....... 72/16 3,357,217 12/1967 Wallace... 72/11 3,253,438 5/1966 Stringer..............,.,,. 72/12 3,332,263 7/1967 Beadle etaln 72/7 3,357,217 12/1967 Wallace et a1, 72/8 OTHER REFERENCES 1960 Iron and Steel Engineer Yearbook," pp. 1311- 140 1961 Iron and Steel Engineer Yearbook, pp. 99 l 997 1964 Iron and Steel Engineer Yearbook," pp. 53 762 1957- 1958 Journal of the lntitute of Metals. pp 289- 302.

Primary Examiner-Milton S. Mehr Attorneys-F. H. Henson and R. G. Brodahl ABSTRACT: A screwdown recalibration system is provided for use in a computer-controlled rolling mill. The computer uses the difference between gauge directly calculated by mass flow concepts and gauge calculated from a measured screwdown unloaded roll opening and roll force at each stand to recalibrate the screwdowns for that stand. The system is adaptable to either providing infonnation to an automatic roll force gauge control system according to a predetermined schedule or to function as an online recalibration scheme which is an integral part of a more comprehensive r011 force gauge control system.

PC|\ sens oovm comnoL SD SCREW POSITION osrscron "warm."

CONTROL SPD SDn SCREW POSITION E DETECTOR I I i 1" x n ev x W 1 I2 I It J LC c I1 1 TI 12 2 r fln 20 4/ A0 V C? k LL T 52:2 6)- 6 LL cu E L n so" SPEED swszo SPEED CONTROL CONTROL onTROL COMPUTER CONTROL SCREWDOWN OFFSET SYSTEM AND METHOD FOR IMPROVED GAUGE CONTROL This application is a continuation of Ser. No. 677,308, filed Oct. 23, 1967, now abandoned.

CROSS-REFERENCE TO RELATED APPLICATION Reference is made to the following: U.S. Pat. No. 3,357,217 entitled Slave Gauge Control System For A Rolling Mill by 1. Wallace.

BACKGROUND OF THE INVENTION The present invention relates to metal-rolling mills and more particularly to calibration of the rolling mill in cooperation with roll force gauge control systems and methods which are used to operate such mills.

in the operation of a metal reversing or tandem rolling mill, both the unloaded roll opening and the speed for each tandem mill stand or for each reversing mill pass are set up either by an operator or by a computer to produce successive workpiece (strip or plate) reductions resulting in an on-gauge finished work product. it may be assumed that the loaded roll opening at a stand equals the stand delivery gauge since there is little or no elastic workpiece recovery.

Because the setup conditions may be in error and, in any event, since certain mill parameters affect the stand loaded roll opening during rolling and after setup conditions have been established, a stand gauge control system must be em ployed to closely control the stand delivery gauge. Thus, at the present state of the rolling mill art and particularly the steelrolling mill art, a stand gauge control system is normally used for a reversing mill stand and for predetermined stands in tandem-rolling mills.

More particularly, the well-known gauge meter or roll force system has been widely used to produce stand gauge control in metal-rolling mills. Moreover, in tandem hot steel strip-rolling mills and reversing plate mills experience has demonstrated that roll force control is particularly effective. Earlier publications and patents such as U.S. Pat. No. 2,726,54l issued Dec. 13, I955 to R. D. Sims described the theory upon which operation of the roll force system is based.

Briefly, the roll force gauge control system uses Hooke's law in controlling the screwdown position at a rolling stand, i.e., the loaded roll opening under workpiece rolling conditions equals the unloaded roll opening (screwdown position) plus the mill spring stretch caused by separating force applied to the rolls by the workpiece. To embody this rolling principal in the roll force gauge control system, a load cell or other force detector measures the roll separating force. The screwdown position is then controlled to balance roll force changes from a reference or set point value and thereby hold the loaded roll opening at a substantially constant value. The following formula expresses the basic relationship:

h loaded roll opening (output workpiece thickness) S,,= unloaded roll opening (screwdown position) K= the mill spring constant F roll separating force.

Typically, the roll force gauge control system is an analog configuration including analog comparison and amplification circuitry which responds to roll force and screw position signals to control the screwdown position and hold the following equality:

AF= measured change in roll force from a force reference AS controlled change in screwdown position from a screwdown reference. Once the unloaded roll opening and the stand speed setups are determined by either the mill operator or the mill computer for a particular workpiece pass or series of passes, the rolling operation is begun; the screwdowns are then controlled to regulate the workpiece delivery gauge from the reversing mill stand or from each roll force control tandem mill stand.

Since the roll force gauge control system functions without sensing actual stand delivery gauge, a screwdown offset is required for the roll force gauge control screwdowns during rolling operation to correct steady state mill delivery gauge errors which stem from various causes. By screwdown offset, it is herein meant to refer to the change in screwdown position made to correct a gauge error which is caused or expected to be caused by a single mill variable or by a combination of mill variables, and which is uncorrectable or inadequately correctable by internal roll force gauge control operations alone. By steady state gauge error it is meant to refer to an error which is correctable by screwdown offset.

One mill condition which will cause a steady state gauge error is an incorrect setup wherein the screwdown position and the stand speed setup at a particular stand result in a headend stand delivery gauge which may not equal the headend gauge predicted from the setup values. lf the roll force control uses a head-end lock-on roll force reference, the stand is roll force controlled to continue rolling the actual head-end gauge unless the screwdowns are externally offset to correct steady state gauge error. Moreover, errors in the determination of the setup parameters of the mill may severely affect the accuracy of the gauge of the finished workpiece. Thus, it would be highly desirable to have a recalibration system which would monitor errors in the workpiece gauge at one or more stands in a mill and provide an offset value which may be used in the setup for the next workpiece and in the rolling of the present or subsequent workpieces.

A second roll force control system uses an absolute roll force setup reference wherein the stand is roll force controlled to operate from the setup roll force. Under this system, cor rect steady state gauge may be achieved since the initial screwdown position is changed to correct for any initial roll force error. However, use of an absolute roll force setup reference may result in erroneous steady state stand delivery gauge which requires screwdown offset, particularly if the initial screwdown position calibration has drifted or if the mill spring constant has a value different from the value assumed in the setup calculation at the gauge being rolled. Since screwdown calibration drift and/or a changed mill spring constant will directly affect steady state gauge, a screwdown offset may be required if a change occurs in either or both of these variables during workpiece rolling following a correct setup or to provide a better setup from an incorrect setup.

The initial screwdown position calibration is a direct electromechanical measurement which traditionally has only been made at the beginning of the work roll life and, if desired, new initial" calibrations are made at various subsequent time points in the work roll life. One means of recalibration of the work rolls was accomplished by driving the rolls together until the load cell detected a force which was thought to be on the linear portion of the mill spring curve. Since the slope of the mill spring curve equals the negative of the fraction of force divided by distance, it was possible to determine a new recalibration or "zero" point by withdrawing the screws a distance defined by the fraction of force divided by the slope of the mill spring curve. Unfortunately, each recalibration required a time interval which delayed the rolling of the next workpiece and valuable production time was lost. Moreover, no allowance was made for the change in calibration which may occur during the rolling of a workpiece or for that matter between scheduled recalibrations. Any such change would necessarily require a screwdown offset for correction of the roll force gauge control operation.

The most common causes of calibration drift during the rolling operation are a result of changes in roll stand heating, roll wear, stand speed, differential leveling operation of the screwdown for shape control and possibly by changes in other mill conditions. When the initial screwdown calibration does drift, changes occur in the screwdown position at which roll facing occurs thereby making the unloaded roll opening correspondence with screwdown position differ from the initial correspondence by the amount of the calibration drift. Hence,

the actual loaded roll opening, i.e., the actual gauge differs from the expected value calculated by an amount equal to the erroneous calibration. This difference represents a gauge error condition which is correctable by a screwdown offset or more specifically, a screwdown recalibration. In addition, if the mill spring constant changes, the actual loaded roll opening differs from the expected loaded roll opening based on mill stretch which is a function of the erroneous mill spring constant. This gauge error condition is similarly correctable by the determination of a screwdown offset.

Other mill variables may also cause steady state gauge er rors which require corrective screwdown offset. For example, in a hot strip mill where strip temperature rundown tends to cause heavy gauge runup, the stand gauge controls may not react fast enough to correct the running of steady state gauge error and as a consequence screwdown offset is needed to make the correction. In addition, elasticity of the workpiece material can be sufficient to result in slightly heavy workpiece gauge even though the roll force gauge control accurately holds the loaded roll opening at the desired value. Gauge heaviness of this kind would be due to the fact that the workpiece reduction is largely plastic and at least partially elastic, and it may require corrective screwdown offset in those cases where it has relative significance. In the case of mill acceleration or deceleration, steady state gauge apparently changes or tends to change as a result of transient changes in stand entry and exit workpiece tension values which are reflected in rapid roll force changes that cannot be tracked by the roll force control. Moreover, it is possible that rate of workpiece speed change could cause transient calibration changes or transient mill spring constant changes or otherwise have an effect on gauge independent of roll force. In any event, screwdown offset is required for steady state correction of gauge error effects produced by a sustained workpiece speed change rate.

To provide state gauge error correction, the well known monitor gauge control system is usually employed to product screwdown offset for the roll force controls. In the monitor system, an X-ray or other radiation gauge is placed at one or more predetermined process points and usually at a single process point following the delivery end of the mill in order to sense actual delivery gauge after a workpiece transport delay from the point in time at which the actual delivery gauge is produced at the preceding stand or stands. The monitor system compares the actual delivery gauge with the desired delivery gauge and develops an analog feedback control signal to adjust the operation of the reversing mill stand roll force gauge control system or one or more predetermined tandem mill roll force gauge control systems to supply desired steady state mill delivery gauge. In this manner, the conventional monitor system provides for correction of steady state gauge errors which are caused or which are tending to be caused by a single mill variable or by a combination of mill variables.

In operator controlled mills, some steady state gauge correcting load can eventually be taken off the monitor system by screwdown recalibration through adjustment of the individual stand gain setting, etc., between the workpiece passes if steady state gauge error tends to exist along the entire workpiece and persists from workpiece to workpiece. In this manner, some reduction is achieved in the length of off-gauge workpiece otherwise associated with monitor transport delay. Similarly, corrective monitor system operation caused by head-end gauge errors can be reduced by changes in the operator setup from workpiece to workpiece or more accurately and more quickly by programmed updating of the mill setup program if a mill setup computer is employed.

Experience with conventional roll force gauge control systems has demonstrated that the achievement of fast and accurate mill delivery gauge control with stability is a complicated and very difficult task under the wide variety of rolling conditions and types and sizes of workpieces or strips encountered in metal-rolling mills. It is particularly desirable that stand gauge control be performed rapidly since even a relatively short time response period can result in a substantial length of off-gauge although not necessarily rejectable gauge work product when high delivery speeds are employed. For example, in tandem hot strip metals, strip delivery gauge accuracy is materially affected by the speed and the stability of stand gauge control because these factors determine the length of strip over which a gauge error persists. Delivery and stand gauge accuracy is, of course, also dependent upon fac tors such as transport delay, accuracy of sensors, and so forth.

The mill spring curve has typically and usually justifiably been treated as having a constant and uniform linear slope and is more commonly referred to as the mill spring line. Actually, at lower force levels which typically have been encountered infrequently in mill use, the spring curve is nonlinear and the spring constant accordingly varies over the nonlinear portion of the curve.

It is also significant that the spring constant corresponding to the slope of the linear part of the spring curve is subject to change during mill operation. On the basis of present knowledge, the slope of the linear portion of the spring curve can be changed (perhaps as much as +l0 percent) both by individual changes or combinations or changes in certain mill parameters including the backup roll diameter and the workpiece width and by a steady state gauge error.

Moreover, any change in the slope of the mill spring curve and resultant steady state gauge error can be resolved by a recalibration system which determines gauge as a function of unloaded roll opening and mill stretch. That is, by moving the unloaded roll opening and thereby changing the relative posi tion of an assumed mill spring curve for which the assumed slope is in error, a gauge can be determined under a particular roll force condition which would exactly match the gauge at another unloaded roll, opening for which a correctly assumed mill spring slope might otherwise be used.

In summary, it is apparent that significant advances in both the technology and commercial embodiments of roll force gauge control systems has been achieved. Yet, to insure the rolling of an accurate workpiece throughout its entire length, it is necessary that these roll force gauge control systems be provided with accurate mill parameters both at the setup and during the rolling of a workpiece. Traditional systems for re calibration have been only partially successful in determining accurate mill parameters requisite to roll force gauge control systems. As such, many areas for improvement exist not only in the recalibration systems themselves, but also in the length of cycle time required for a calibration and in developing efficient communication linkages with the roll force gauge control system.

SUMMARY OF THE INVENTION It is, therefore, a general object of the present invention to provide a new and improved recalibration system for at least one rolling stand in a rolling mill operating under a roll force gauge control system.

A further object of the present invention is to provide a new and improved recalibration system for at least one rolling stand in a rolling mill operating under a roll force gauge control system wherein any sustained calibration error will be removed after the rolling of several workpieces.

Another object of the present invention is to provide a new and improved rolling mill operative under a roll force gauge control system wherein recalibration information is a result of rolling a workpiece as opposed to a separate duty cycle.

Yet, a further object of the present invention is to provide a new and improved rolling mill operative under a roll force gauge control system wherein the recalibration information from the rolling of one workpiece is used to provide an improved setup for the rolling of the next workpiece.

An additional object of the present invention is to provide a new and improved recalibration system for a rolling mill operative under a roll force gauge control system wherein the recalibration may be independent of any change in measured roll force.

A still further object of the present invention is to provide a new and improved recalibration system for a rolling mill operative under a roll force gauge control system which compensates for nonlinearity in the mill spring curve.

Still another object of this invention is to provide a new and improved recalibration system for a rolling mill operative under a roll force gauge control system wherein each stand of the rolling mill is individually recalibrated on a predetermined time cycle.

Yet, a further object of the present invention is to provide a new and improved recalibration system for a rolling mill which cooperates with a roll force gauge control system by providing an offset factor representing the online relative change in calibration over a predetermined time period.

In accordance with the principles of the present invention the delivery gauge for the last stand is measured by an X-ray gauge and a comparison is made between this measured gauge and the gauge calculated using the screwdown unloaded roll opening and the measured roll force. Since the mass flow for the rolling mill is substantially constant, precise measurement of stand speed can be used to calculate the delivery gauge from any of the other stands by using the following equation:

(measured delivery gauge) I: (gauge,,)(stand speed,,)/(stand speed where:

i= stand number being measured n number of last stand. This measure delivery gauge can then be compared with the "calculated gauge" by measuring the screwdown roll separating force in the following equation: (calculated delivery gauge):

(unloaded roll opening, (measured force),/K where:

K the mill spring constant at the force measured. The difference between the measured and actuated delivery gauge at each stand is used to recalibrate the screwdowns of that stand to provide an improved operating condition. The recalibration may be used to develop more accurate setup conditions for subsequent workpieces or it may be a part of an online roll force gauge control system.

DESCRIPTION OF THE DRAWINGS FIG. I is a diagrammatic representation of a rolling mill showing the necessary inputs and outputs in a system for screwdown recalibration based on the principles of the present invention.

FIG. 2 is a diagrammatic representation of the logic flow required to provide for recalibration of the screwdowns.

FIG. 3 is a diagrammatic representation of an online recalibration system in cooperation with a roll force gauge control system; and

FIG. 4 is a graph of roll force versus workpiece thickness roll opening showing the mill spring curve and the metal plastic deformation curve for a particular rolling stand.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, a diagrammatic representation of a tandem rolling mill is shown in connection with the inputs and outputs relative to a preferred embodiment of the present invention and is designated by the numeral 10. It should be noted that the principles of this invention may be applied as well to a single stand reversing mill having comparable inputs and outputs. A multistand rolling mill is shown comprised ofa plurality of stands 5,, S S,,. An unfinished workpiece I4 is fed into the first stand S of the mill and necessarily reduced in gauge stand by stand to a desired finished gauge final product which is then coiled by a coiler 16. Each stand comprises a set of cooperating rolls R R R, respectively which effects a certain amount of thickness reduction in the workpiece I4 as it passes therebetween.

The rolls R of stand S are driven by a direct current motor M, which is connected to drive the rolls R through suitable mechanical coupling means. The other stands 8,, S, are similarly provided with individual drive motors M M, and mechanical couplings to operate the rolls R,, R, respectively. Each of the drive motors are provided with an individual speed controller SC SC,, SC, which is responsive to a control signal from a computer control system I8 to be described later. Mechanically connected to each of the drive motors M,, M,, M, is a respective tachometer T T T, which provides signals responsive to the speed of its respective drive motors to the computer control system I8.

As the workpiece is reduced in thickness at a rolling stand, the force required to effect the reduction is transmitted through the work rolls and is measured by a load cell. These load cells are designated by LC LC LC, and provide respective input signals to the computer control system 18. The delivery gauge from the last stand S, is measured by an X- ray gauge 20 which provides an input signal proportional thereto to the computer control system I8.

Associated with each of the roll sets R,, R R, is a respective screwdown SD 8D,, SD, which acts on the respective rolls to produce an opening for the workpiece which presumably will provide the correct delivery gauge from that stand. Connected electrically to each of the screwdowns is a screw position detector SDD,, SDD SDD, which is responsive to the relative position of the screwdowns 8D,, 8D,, SD,,. Movement of the screwdowns is effected through operation of respective screwdown motors SDM SDM SDM, which through a mechanical connection lower or raise the screwdown and consequently alter the unloaded roll opening. The driving signal for the screwdown motors is derived from respective screwdown-positioning controls PC,, PC PC,. which are responsive to signals from said computer control system 18.

Generally, the computer control system [8 may comprise any of the known digital process computers such as the Westinghouse Prodac 250 under the control of an externally provided programming system. The inputs and outputs of the computer control system 18 are appropriately interfaced with the process controlled hardware through conventional and compatible analog to digital and digital to analog converters (not shown). Other equipment associated with rolling mills such as tension controllers between stands, water sprays, temperature sensors, gauge control systems, etc. are not shown for purposes of ease of illustration.

Recalibration of the rolling stand in accordance with principles of the present invention may occur either as required and determined by an operator or computer or as an integral part of an online computer gauge control system. In the former case, recalibration occurs at necessary and convenient times such as for the startup of a mill, following a wreck, or alter a roll change. In general, the most opportune time would be immediately after mill is full, i.e., metal is being rolled in all rolling stands. In the latter instance, however, recalibration is done on a continuing basis and provides a necessary correction or offset factor to a roll force gauge control system which may be utilized both during the rolling of a workpiece and for the setup for the rolling of a subsequent workpiece. For purposes of definition, the term offset factor" is herein defined as a correction factor which represent the amount of screwdown offset necessary because of a change in calibration.

The flow diagram as shown in FIG. 2 sets forth the basic principles of a programming system compatible with a com puter control system 18 and the rolling mill I0 of FIG. 1. Although, the programming system provides some degree of calibration control with the X-ray gauge 20 off, it is desirable for maximum effectiveness that both the X-ray gauge 20 be operating and that the workpiece 14 be present at all stands.

Generally calibration control is achieved by comparing the calculated exit gauge at each stand as determined from measured roll force and screwdown unloaded roll opening with the actual gauge at each stand and causing or permitting a recalibration of the screws based on all or a portion of this difference. That desired gauge may be different from gauge as determined by X-ray is of no avail as much as this discrepancy is reconciled and the screwdowns adjust by an absolute roll force gauge control system. It is, moreover, the function of this invention to cause the actual and calculated gauge to coincide and the function of a roll force gauge control system and if necessary a monitor system to cause the actual, the calculated gauge and the desired gauges to coincide. The combined result of both systems is thus to force the actual or Xray gauge to that of the desired gauge and thereby provide a more perfect and on-gauge end product.

At a predetermined time during the operation of the mill when a calibration check is desired, the programming system as shown logically in FIG. 2 and using the inputs of HO. 1 is initiated by directing the computer to the start block 200. Block 210 provides initialization of certain counters and accu mulators required in recalibration; the sum of the mass flow (EMF) is set to zero, the stand number counter (i) is set to one and as a second stand counter NP is set to zero. That portion of H6. 2 comprised of blocks 220, 230, 240, and 245 is used to accumulate mass flow and calculate a gauge based on roll force. Block 220 checks to see if stand i or initially if stand l is rolling metal. If the stand is rolling metal, the gauge is then calculated in block 230 according to the following formula:

where:

Hc calculated gauge at stand i S screwdown position from screwdown detector SF'D 0S the offset factor at stand i Fx, the force measured from load cell LC K the mill spring constant at the measured force Fxi AI, correction factor due to nonlinearity of the mill spring line.

In addition, counter NP, equal to the number of stands rolling metal, is increased by one. Also, the sum of the mass flow is accumulated by adding the following to the present mass flow accumulated:

where:

V,= stand speed from tachometer T,

Following these calculations, a check is made in block 240 as to whether all stands have been scanned by determining if stand counter 1' equals n, the total number of stands in the mill. If this condition is not yet satisfied the stand counter i is incremented by one in block 245 and the cycle beginning at block 220 is again repeated. For any stand that is not rolling metal as determined in block 220, all the calculations of block 230 are bypassed. Thus, when all the stands have been scanned as determined by block 240, the following information is known:

1. the calculated gauge at each stand rolling metal, Hq,

2, the sum of the mass flows at each stand rolling metal, (MF, and

3. the number ofstands rolling metal, NP.

Next, the average mass flow AMF is calculated in block 250:

AMF=MFINP whereby the average is based on the number of stands rolling metal.

Block 260 interrogates the X-ray gauge to determine if it is on. If so, then the exit or delivery gauge at the last stand He,. is calculated in block 270 by the following formula:

Hc,,=AMF/V,, where V, stand speed from tachometer T,,.

It should be noted that under certain rolling conditions, the stand speed as measured from a tachometer may not provide an accurate indication of the linear speed of the workpiece as it passes through that stand. Thus, the stand speed V. may need to be modified by a forward slip factor to provide a proper measure of the true linear speed. The calculated last stand exit gauge He, is then in block 280 compared to the actual exit gauge H1, as measured by the X-ray gauge to determine if the measured gauge is within :5 percent of the actual gauge. If so, in block 290, the mass flow MF as calculated from the last stand is determined by:

If, on the other hand, the measured gauge is not within 1-5 percent of the actual gauge, the condition is annunciated at block 285 and at block 295, the mass flow MF is then set equal to the average mass flow AMF as determined at block 250. It should be noted that if the X-ray gauge is not operating as determined at block 260, the mass flow MP is similarly set to the average mass flow AMF in block 295.

The programming system thus far discussed is designated as Part I in that the common purpose of included blocks 200 through 295 is to establish a calculated gauge for each stand He and an accurate mass flow MP. The remainder of the programming system of FIG. 2 is designated Part [I and is used to compute the actual gauge of each stand and a screwdown offset reflecting the difference between actual and calculated gauge.

To begin Part ll, the stand counter i is reset to one in block 300. Block 310 interrogates stand i or initially stand one to determine if it is rolling metal. If so, the actual gauge Hx, is calculated in block 320 by: Hx;=MF/V,. Then the gauge for stand i as measured or calculated in block 230 is compared in block 330 to the actual stand gauge H1, to determine whether the measured gauge He, is within :10 percent of the actual gauge Hx if the measured gauge falls within this range, a new offset factor 0.8] is computed in block 340 according to the following formula:

0S' previous offset factor AOS. change in offset since last calibration.

Block 350 checks to see if the previous offset factor for stand i is zero which means the stand i has just been recalibrated. In this case, in block 360, the offset factor OS, is set equal to the new offset factor just calculated 08,. On the other hand, if the stand has not been recalibrated and the previous offset factor OS is not zero, a new offset factor 08, is calculated in block 365 which is a weighted average of the previous offset factor OS, and the new offset factor 08, calculated in block 340. In either case, once a new offset factor for stand i is determined, the stand counter i is interrogated in block 370 to see if all stands have been scanned. If so, the recalibration cycle is completed at block 390. If not, the stand counter i is incremented by one in block 380 and the offset computation cycle is then repeated for the next stand. it should be noted that when a stand is not rolling metal as determined by the block 310 or the measured gauge He, does not fall within tlO per cent of the actual gauge Hx no offset calculation is made and the next stand is interrogated via blocks 3'10, 380 and 310.

The system just described is particularly adaptable to determining calibration of the rolling mill for setup as an improvement upon the more traditional method of driving the rolls together until the interacting force between them is on the linear portion of the mill spring line and thereafter backing the rolls off a distance equal to the measured divided by the slope of that linear portion of the mill spring curve. indeed, this system may be extended from calibration only upon setup or a roll change to scheduling of calibration between rolling of workpieces to provide updated calibrations on the same work rolls. The present invention, however, is a significant improvement over the traditional method in that no additional time or duty cycle is required to recalibrate work rolls. Rather, the recalibration occurs as a consequence of the normal rolling of the mill and is updated for each workpiece. For example, once the mill is full (the workpiece is present in all stands), the recalibration scheme is initiated which has the capability with a computer to store an offset factor for each stand which is proportional to any miscalibration of the respective rolling stand.

At the completion of rolling a workpiece, the stored offset factors can then be used to recalibrate the screws before the rolling of the next workpiece. In general, calibration or recalibration is mandatory for (l) a new setup, (2) following a wreck and, (3) following a roll change. The system herein described may be used as a substitute or a complement to the existing and traditional calibration techniques; moreover, it may be used as an updating procedure between the rolling of strips on the same setup to provide, in effect, a calibration averaging or, more precisely, a timely and more accurate calibration during the life of a set of work rolls. The offset factors calculated may be used in conjunction with a computer or other online roll force gauge control system to thereby provide a more accurate finished workpiece. In such an application, the rolled gauge is a function of the offset factor and by providing an updated offset factor after each strip a better setup can be achieved and an improved finished workpiece is effected.

Moreover, a further extension of the instant application can be realized by providing an online rather than a strip-by-strip recalibration system. That is, when any change in screwdown is caused or about to be caused by a conventional or other roll force gauge control system which is operated on the basis of Hookes law, an offset factor is determined for each rolling stand and applied by the roll force gauge control system in an online recalibration system in accordance with the principles of this application. FIG. 3 presents in a broad sense a roll force gauge control system that would be compatible with an online recalibration system in a single stand or multistand mill. The procedure is initiated at the start block 400 and proceeds to block 410 wherein a stand counter j is set equal to one. From here Part I of the recalibration as set forth in FIG. 2 is condensed in block 420 which computes an accurate mass flow per stand MF as well as a calculated gauge Hr: for each stand based on a measured roll force value. Block 430 checks to see if stand j is rolling metal and if so the actual gauge Hx, is computed in block 440. If the calculated gauge He, is within ilO percent of the actual gauge Hx, as shown in block 450, a new offset OS, is calculated in block 460. This new offset value is then used in fixing a screwdown position in relation to a predetermined roll force gauge control system shown generally in block 470.

In the roll force gauge control system 470, the screw position required for rolling accurate gauge is equal to a predetermined unloaded roll opening plus a predetermined mill stretch as previously indicated. Further, the offset factor is employed by the roll force gauge control 470 in reflecting the true value of the unloaded roll opening as screw position changes are made to compensate for roll force error. In practice, the online roll force gauge control can itself be embodied in computer form or in some cases it may possibly be a more or less self-contained conventional analog system. In any case, online screwdown recalibration enables more accurate stand gauge to be achieved more accurately.

After completion of the roll force gauge control system function for stand j, it is then necessary to see if all stands have been scanned in block 480. If so, a work cycle has been completed and thus the end block 490. If not, the stand number j is increased by one in block 485 and Part I of the recalibration system is begun for the next sequential scan. It should be noted that in block 450, if the ratio of the calculated gauge Hc, to the actual gauge Hx, does not fall within its predetermined range, then no new offset factor is calculated and the old offset factor is used. A similar consequence exists when the stand being scanned is not rolling metal as deter mined in block 440. It should be noted that the validity ranges used heretofore for determining the validity of calculated values is intended simply by way of example and that these ranges may be changed to any values consistent with the degree of control desired.

Referring to FIG. 4, the mill spring curve ML and the plasticity curve are shown on a graph of roll force versus thickness to illustrate a condition wherein a recalibration is required. The mill spring curve ML is shown for a particular stand in the mill. This line is linear and has a constant slope at higher forces and nonlinear with a varying slope at lower forces. A plasticity curve P relative to the material hardness intersects the mill spring curve ML at a point T which, when projected to the X axis, represents the calculated exit stand gauge He. The relative position of the mill spring curve ML on the X axis is determined by the sum of the unloaded screwdown opening S0 and any offset value US as a result of previous recalibrations. Because most metal rolling is done at higher force levels, the aforementioned sum is equal to the intersection of the slope of the linear portion of the mill spring curve ML with the X axis as shown in FIG. 4.

In practice and as previously described, a recalibration or a change in offset is determined by the difference between the calculated gauge He and the actual gauge Hx. Any offset factor previously used is stored in memory of the computer control system 18 to FIG. I. As metal is rolling in the stand, a measured force Fx is received as input from the load cell associated with the stand. The intersection of this force P1: with the mill spring curve ML at point T then determines the calculated gauge Hc. Since the mill spring curve ML is a function of measured force Fx, by substituting the force measured E: into the equation for the mill spring curve the calculated or measured gauge is determined. The equation for determining the calculated gauge He is given by the following formula:

H =S +OS K 1 F )Al where each of the terms is shown graphically in FIG. 4.

The unloaded roll opening plus the previous offset (S,,+0S') is given by the intersection of the linear slope of the mill spring line ML with the X axis. The product of the inverse of the slope of the mill spring curve at the point of intersection T and the measured force F, provide the additional data required to calculate a measured gauge H,. If the point of intersection T is on the nonlinear portion of the mill spring curve ML, a factor A! equal to the difference it intercepts with the X-axis of the slope K and the slope of the linear portion of the mill spring curve ML is subtracted from the previous sum to compensate for the nonlinearity. Suppose that the X-ray gauge determines that the actual gauge H, is heavier than calculated. The effect of this condition is that because of roll wear, roll heating, increased hardness of the strip and any of a number of other reasons, the roll force measured is actually detecting that the intersection T should be at T. Thus the plasticity curve should be at P and the mill spring curve at ML. The horizontal parallel movement of the mill spring curve is over a horizontal distance AOS such that the intersection of the slope of the mill spring curve with the X axis is at:

The term 08 then represents the screwdown offset necessary to bring the screws into calibration either on the next workpiece or on the same workpiece if an online roll force gauge control system is being used.

Since additional changes not herein specifically referred to may be made in the above described system, and different embodiments of the invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative only and not in a limiting sense.

I claim as my invention:

I. An apparatus for controlling a rolling mill having at least a first rolling stand followed by a second rolling stand and being operative to reduce the thickness of a workpiece, said rolling mill having a screwdown system for at least said first rolling stand, the combination of means responsive to the roll force of said first rolling stand and the unloaded roll opening of said first rolling stand for determining a calculated roll force delivery gauge for said first rolling stand,

means responsive to the actual delivery gauge and the operating speed of said second rolling stand and responsive to the operating speed of said first rolling stand for determining the mass flow delivery gauge for said first rolling stand, and

means responsive to the difference between said calculated roll force gauge and said mass flow delivery gauge for determining a screwdown system recalibration offset for said first rolling stand.

2. The apparatus of claim 1, including computer means operative with said first rolling stand for providing mill spring curve information to be utilized in determining said calculated roll force delivery gauge for said first rolling stand. 3. The apparatus of claim 2, with said screwdown system icing operative to adjust the roll opening of the rolling stand issociated with said screwdown system, and with said re- :alibration offset being determined after the rolling of a given vorkpiece to provide unproved setup conditions for the 'olling of a subsequent workpiece.

4. The apparatus of claim 2 operative within a roll force gauge control system for said first rolling stand to provide an )nline and updated screwdown offset for the latter roll force gauge control system during the rolling ofa given workpiece.

S. The apparatus of claim 4, with said computer means in- :luding a control program for determining a predetermined validity status relative to said actual delivery gauge and said nass flow delivery gauge for said first rolling stand before providing an updated screwdown offset during the rolling of a given workpiece.

6. The apparatus of claim 1, with said computer means compensating for nonlinearity in the mill spring curve for determining a predicted gauge from said first rolling stand.

7. The apparatus of claim 2, with said computer means including a control program operative to effect a comparison between the actual mass flow delivery gauge and the calculated roll force delivery gauge for said first rolling stand and to provide an updated screwdown offset for said first rolling stand when said comparison falls within a predetermined validity range.

8. ln apparatus for controlling a rolling mill having a plurality of rolling stands operative to reduce the thickness of a workpiece, said rolling mill having a screwdown system for each of said rolling stands, the combination of means responsive to the roll force of each rolling stand prior to the last of said rolling standings and responsive to the unloaded roll opening of that same rolling stand for determining a calculated roll force delivery gauge for each rolling stand prior to said last rolling stand means responsive to the actual delivery gauge and the operating speed of said last rolling stand and responsive to the operating speed of each rolling stand prior to said last stand for determining the mass flow delivery gauge for each rolling stand prior to said last rolling stand, and

means responsive to the difference between said calculated roll force delivery gauge and said mass flow delivery for each rolling stand prior to said last rolling stand for determining a screwdown system recalibration offset for each rolling stand prior to said last rolling stand.

9. The apparatus of claim 8, including means for determin ing the average mass flow delivery gauge for at least one of said stands in relation to the number of said rolling stands that are operative to reduce the thickness of said workpiece.

10. The apparatus of claim 9, with the screwdown system for each rolling stand being operative to adjust the roll opening of the associated rolling stand, and including computer means operative with each of said rolling stands for providing mill spring curve information to be utilized in determining the calculated roll force delivery gauge for each of said rolling stands.

ll. The apparatus of claim 10, with said computer means being included within a roll force gauge control system and providing online and updated screwdown offset for each of said rolling stands during the rolling of a given workpiece.

12. [n a system for controllmg a rolling mill having at least one rolling stand operative to reduce the thickness of a workpiece under the control of a roll force gauge control system, a screwdown recalibration system comprising the combination of:

means for detecting the roll force at said rolling stand and for providing a roll force output signal representing the roll force detected at said stand;

means for detecting the unloaded roll opening at said rolling stand and providing a position output signal corresponding to the unloaded roll opening of said stand;

means for determining the actual workpiece gauge on the exit side of said rolling stand;

means responsive to said roll force output signal and said position output signal for determining a calculated stand delivery gauge;

means responsive to the difference between said calculated gauge and said actual workpiece gauge for effecting a screwdown offset factor for said rolling stand,

said determining and effecting means include computer means operative to determine the calculated stand gauge from a stored mill spring curve and from the aforementioned roll force and position output signals,

, wherein said computer means includes a programming system which provides for using the initial offset factor in its entirety upon a change in mill setup to provide a screwdown offset and thereafter weighing further offset factors on the same setup according to a predetermined ratio with the previous offset factor to provide an updated screwdown offset.

13. The method of controlling a rolling mill having-at least one earlier rolling stand and one subsequent rolling stand operative to reduce the thickness of a workpiece under the control of a roll force gauge control system, said rolling mill having a roll opening adjustment system for each of said rolling stands, the steps of establishing a calculated roll force delivery gauge for at least said one earlier rolling stand in accordance with the roll force of that same rolling stand and the unloaded roll opening of that same rolling stand, establishing the mass flow delivery gauge for at least said one earlier rolling stand in accordance with the operating speed of that same rolling stand in relation to the operating speed of said one subsequent rolling stand, and

establishing a recalibration offset factor for said roll opening adjustment system of at least said one earlier rdlling stand in accordance with a predetermined comparison between said calculated roll force delivery gauge and said mass flow delivery gauge for that same earlier rolling stand.

14. The method ofclaim 13, including the steps of establishing a validity range for said predetermined comparison between sad calculated roll force delivery gauge and said mass flow delivery gauge for at least said one earlier rolling stand,

and adjusting said roll opening of at least said one earlier rolling stand in accordance with said comparison only when said comparison falls within said validity range.

IS. The method ofclaim 13, including the step of establishing the roll opening of at least said one earlier rolling stand in accordance with said offset factor for that same rolling stand, with said offset factor being utilized in its entirety upon a change in the setup of that same rolling stand and thereafter, weighing further established offset factors for that same rolling stand in accordance with a predetermined relationship with the previous offset factor to provide an updated offset factor. 

1. An apparatus for controlling a rolling mill having at least a first rolling stand followed by a second rolling stand and being operative to reduce the thickness of a workpiece, said rolling mill having a screwdown system for at least said first rolling stand, the combination of means responsive to the roll force of said first rolling stand and the unloaded roll opening of said first rolling stand for determining a calculated roll force delivery gauge for said first rolling stand, means responsive to the actual delivery gauge and the operating speed of said second rolling stand and responsive to the operating speed of said first rolling stand for determining the mass flow delivery gauge for said first rolling stand, and means responsive to the difference between said calculated roll force gauge and said mass flow delivery gauge for determining a screwdown system recalibration offset for said first rolling stand.
 2. The apparatus of claim 1, including computer means operative with said first rolling stand for providing mill spring curve information to be utilized in determining said calculated roll force delivery gauge for said first rolling stand.
 3. The apparatus of claim 2, with said screwdown system being operative to adjust the roll opening of the rolling stand associated with said screwdown system, and with said recalibration offset being determined after the rolling of a given workpiece to provide unproved setup conditions for the rolling of a subsequent workpiece.
 4. The apparatus of claim 2 operative within a roll force gauge control system for said first rolling stand to provide an online and updated screwdown offset for the latter roll force gauge control system during the rolling of a given workpiece.
 5. The apparatus of claim 4, with said computer means including a control program for determining a predetermined validity status relative to said actual delivery gauge and said mass flow delivery gauge for said first rolling stand before providing an updated screwdown offset during the rolling of a given workpiece.
 6. The apparatus of claim 2, with said computer means compensating for nonlinearity in the mill spring curve for determining a predicted gauge from said first rolling stand.
 7. The apparatus of claim 2, with said computer means including a control program operative to effect a comparison between the actual mass flow delivery gauge and the calculated roll force delivery gauge for said first rolling stand and to provide an updated screwdown offset for said first rolling stand when said comparison falls within a predetermined validity range.
 8. In apparatus for controlling a rolling mill having a plurality of rolling stands operative to reduce the thickness of a workpiece, said rolling mill having a screwdown system for each of said rolling stands, the combination of means responsive to the roll force of each rolling stand prior to the last of said rolling standings and responsive to the unloaded roll opening of that same rolling stand for determining a calculated roll force delivery gauge for each rolling stand prior to said last rolling stand means responsive to the actual delivery gauge and the operating speed of said last rolling stand and responsive to the operating speed of each rolling stand prior to said last stand for determining the mass flow delivery gauge for each rolling stand prior to said last rolling stand, and means responsive to the difference between said calculated roll force delivery gauge and said mass flow delivery for each rolling stand prior to said last rolling stand for determining a screwdown system recalibration offset for each rolliNg stand prior to said last rolling stand.
 9. The apparatus of claim 8, including means for determining the average mass flow delivery gauge for at least one of said stands in relation to the number of said rolling stands that are operative to reduce the thickness of said workpiece.
 10. The apparatus of claim 9, with the screwdown system for each rolling stand being operative to adjust the roll opening of the associated rolling stand, and including computer means operative with each of said rolling stands for providing mill spring curve information to be utilized in determining the calculated roll force delivery gauge for each of said rolling stands.
 11. The apparatus of claim 10, with said computer means being included within a roll force gauge control system and providing online and updated screwdown offset for each of said rolling stands during the rolling of a given workpiece.
 12. In a system for controlling a rolling mill having at least one rolling stand operative to reduce the thickness of a workpiece under the control of a roll force gauge control system, a screwdown recalibration system comprising the combination of: means for detecting the roll force at said rolling stand and for providing a roll force output signal representing the roll force detected at said stand; means for detecting the unloaded roll opening at said rolling stand and providing a position output signal corresponding to the unloaded roll opening of said stand; means for determining the actual workpiece gauge on the exit side of said rolling stand; means responsive to said roll force output signal and said position output signal for determining a calculated stand delivery gauge; means responsive to the difference between said calculated gauge and said actual workpiece gauge for effecting a screwdown offset factor for said rolling stand, said determining and effecting means include computer means operative to determine the calculated stand gauge from a stored mill spring curve and from the aforementioned roll force and position output signals, wherein said computer means includes a programming system which provides for using the initial offset factor in its entirety upon a change in mill setup to provide a screwdown offset and thereafter weighing further offset factors on the same setup according to a predetermined ratio with the previous offset factor to provide an updated screwdown offset.
 13. The method of controlling a rolling mill having at least one earlier rolling stand and one subsequent rolling stand operative to reduce the thickness of a workpiece under the control of a roll force gauge control system, said rolling mill having a roll opening adjustment system for each of said rolling stands, the steps of establishing a calculated roll force delivery gauge for at least said one earlier rolling stand in accordance with the roll force of that same rolling stand and the unloaded roll opening of that same rolling stand, establishing the mass flow delivery gauge for at least said one earlier rolling stand in accordance with the operating speed of that same rolling stand in relation to the operating speed of said one subsequent rolling stand, and establishing a recalibration offset factor for said roll opening adjustment system of at least said one earlier rolling stand in accordance with a predetermined comparison between said calculated roll force delivery gauge and said mass flow delivery gauge for that same earlier rolling stand.
 14. The method of claim 13, including the steps of establishing a validity range for said predetermined comparison between sad calculated roll force delivery gauge and said mass flow delivery gauge for at least said one earlier rolling stand, and adjusting said roll opening of at least said one earlier rolling stand in accordance with said comparison only when said comparison falls within said validity range.
 15. The method of claim 13, including the step of establishing the rOll opening of at least said one earlier rolling stand in accordance with said offset factor for that same rolling stand, with said offset factor being utilized in its entirety upon a change in the setup of that same rolling stand and thereafter, weighing further established offset factors for that same rolling stand in accordance with a predetermined relationship with the previous offset factor to provide an updated offset factor. 